Japanese unexamined patent application publication No. JP-A-2007-163884 describes a related art image forming apparatus. The related art image forming apparatus such as an electro-photographic color printer is provided with a fixing unit for fixing toner transferred to a sheet onto the sheet. The fixing unit includes a heating roller and a pressing roller placed in a press state against the heating roller. The sheet with toner transferred thereto is conveyed through the nip between the heating roller and the pressing roller. While the sheet passes between the heating roller and the pressing roller, the toner is fixed onto the sheet by heating and pressing.
To control the fixing temperature, the fixing unit is provided with a temperature sensor for detecting the surface temperature of the heating roller or the pressing roller. For example, a fixing unit in the related art image forming apparatus has a noncontact thermistor placed at a predetermined distance from the surface of a pressing sensor. Output of a heater provided in the pressing roller is controlled so that the temperature detected by the noncontact thermistor becomes a predetermined target fixing temperature.